The present invention relates generally to the operation of high intensity discharge lamps and, more particularly, to a solid state ballast for operation of such lamps.
High intensity discharge (HID) lamps such as mercury, metal halide and high pressure sodium lamps are popular sources of light because of their high efficiency in converting electrical energy into light. Unfortunately, discharge lamps typically are operated through ballast circuits which are very inefficient. A ballast circuit is connected between a power source and a lamp to provide a high initial voltage to start the lamp and then to limit current through the lamp to safe levels during continued operation.
The most common conventional ballast circuit includes a transformer having a variably coupled secondary winding such that the magnetic coupling of the secondary winding is not constant. Thus, the voltage on the secondary winding can vary according to the load which it is driving. Effectively, the voltage varies to maintain a substantially constant current through the secondary circuit. Such operation is very favorable for the control of discharge lamps because the constant current maintains stable power delivery to the lamps and prevents the natural tendency of the lamps to "run-away" upon ignition when the impedance goes to virtually zero. The lamp will eventually equilibrate based on the coupling strength that was built into the transformer.
Such conventional ballasts are represented as operating lamps at constant wattage or power based on the constant current and the ideal that the voltage across a lamp also remains constant. However, lamp voltage increases at a rate of about one volt per thousand hours of operating time such that lamp power consumption creeps upward with the age of the lamp. Power consumption can increase 20% over the life of a lamp.
Another problem with such conventional ballasts is the wide variations in power level at which a lamp equilibrates. The variations in equilibration power is due to the inability to precisely set the magnetic path strengths within the ballast transformer and can result in operating power level errors of up to 25% of nominal. Once a transformer has been made, it is not possible to change the power output level of a lamp operated by a ballast including the transformer, i.e. a lamp is either on and operated at the transformer defined power level or the lamp is off. This lack of control effectively eliminates lighting as a variable in energy management strategies.
Still another problem with such conventional ballasts is noise generated by lamp operation. Since the core of a transformer of a conventional ballast is constructed of steel laminations, as the transformer ages the laminations loosen and can produce high noise levels. Noise generation is compounded by the nature of HID lamps which draw current in pulses centered around the center 30% of a driving sine wave power signal. The current pulsing causes rapid flux changes in the core and aggravates vibration of any loose laminations to produce load buzzing not ordinarily associated with transformer operation. Such transformers also tend to be large, bulky and heavy even for low lamp power levels.
While a variety of less conventional ballast circuit configurations have been employed in the prior art including electronic circuitry, controlled core saturations and others, none have been totally acceptable for overcoming the problems encountered in conventional transformer ballast circuits.
Accordingly, there is a need for an improved ballast circuit having higher efficiency, substantially constant or controllable lamp power over the life of a lamp, smaller size with reduced weight and low noise operation.